Combined amplitude-modulation and frequency-modulation receiver stage



Oct. l; 1946. J. W.'HUFF 2,408,644

COMBINED MPLITUD'E-MODULATION AND FREQUENCY-MODULTION RECEIVER STAGE I vFiled'Jply 24, 1944 I E9n/.7% @mf/17W ATTORNEYS Patented Oct. 1, 1946man CORIBINED ANIPLITUDE-MODULATION AND y FREQUENCY MODULATION RECEIVERSTAGE John W. H ul, Chicago, Ill., assigner, by mesne assignments, toBelmont RadiofCorporation, Chicago, Ill., a corporation of vIllinoisApplication July 24, 1944,. Serial No. 546,298

Y 1 y A This invention relates to combined amplitudemodulation andfrequency-modulation receiver stages and to a receiver embodying thesame and particularly to a receiver of the type designed with a view tomaximum simplicity for marketing in the lower price ranges.

In the prior art receivers designed for the reception of the standardbroadcast band of approximately 500-1500 kilocycles and thefrequency-modulation band of 41-50 megacycles, it has been the practiceto provide an amplitude-modulation signal-transmitting channel includinga diode amplitude-modulation detector, a frequency-m'odulationVsignal-translating channel, at

least in part distinct from the amplitude-modulation channel andincluding one or more limiters followed by a frequency discriminator ordetector, and an'audio-frequency system selectively energized fromk theamplitude-modulation channel or the frequency-modulation channel. Thisuse of distinct amplitude-modulation and frequency-modulation channelsincreases` the complexity and cost oi the receiver and has limited thelower price ranges within which thisy Atype of receiver could bemarketed.

` It is an object of the invention, therefore, to provide a new andimproved combined amplitudemodulation and frequency-modulation receiverstage for selectively effecting detection of a receivedamplitude-modulation signal or acting as ajlimiter for areceivedfrequency-modulation` signal.

It is another` object of the invention to provide an improvedl receiveradapted selectively to receive amplitude-modulated signals orfrequencymodulated'signals and including a combined am- In accordancewith the invention, in a wave- 'i signal receiver4 adapted selectivelyto receive amplitude-modulated signals orfrequency-modulated signals, adual-purpose signal-translating stage comprises a vacuum tuber having aplurality of electrodes, including a signal input electrode and a signaloutput electrode, and an input circuitior applying received signals to.the input electrode. The stage also includes a. frequency discriminatorcoupled to the output electrode of 7 claims. (erecto-,20),

operate as a frequency-modulation limiter. YThe stage. also includesswitch means operable ina first position to establish the 'first-namedcircuit connections and to connect the output circuit to the amplitudemodulation detector and operable in a second position to establish rthesecondnamedpcircuit connections and to connect the output'circuit to thediscriminator.

Also in accordance with the invention, a wave signal receiveradaptedselectively to receive amplitude-modulated signals orfrequency-modulated signals comprises, in the order named, aradio-frequency selecting and converting means for deriving from aselected received radio-frequency signal an intermediate frequencysignal, a dual-purpose signal-translating stage of the type described,and a signal-reproducing means coupled to the signal-frequency outputcircuit of the stage. For a rbetter understanding of the invention,together, w'ithv other and further objects thereof, reference is" had toythe following description taken in connection n with the accompanyingdrawing 'while'its scope will be pointed out in the appended claims. y n

vReferring now to the drawing, the single figure is a circuit diagram,partly schematic, of a com-v plete, superheterodyne receiver embodyingthe combined amplitude-modulation and frequencymodulation receiver stageof the invention.

, Referring now to the single figure 'of the drawing, there isrepresented, partially schematically, the circuit :of a completesuperheterodyne wavesignal receiver adapted selectively to receiveamplitude-modulated signals or frequency-modulated signals. Thisreceiver comprises a. radiofrequency amplifying and selecting unit lllhaving its input circuit connected Ato an antenna circuit Il anditsoutput circuit connected to an oscillator modulator I2,these twounits being effective to.. derive from a selected receivedradio-frequency signal -an intermediate-frequency signal from eithertheamplitude-modulation range or the frequency-modulation. range. The unitsl0 and i2 may be of conventional design with suitable-range switchesIlla, l2a, respectively, for selectively translatingVamplitude-modulated sign'alsv or-'frequency-modulated signals. Thereceiver also includes an"intermediate-frequency the vacuum tube, asignal-frequency output cirselector and amplifier, I3 coupled' to theoscillator modulator I2for translating'the intermediate-'- 3 cuits Ma,Illb tuned to transmit the frequencymodulated intermediate-frequencysignal in series with the primary and secondary tuned circuits Mc, Id,respectively, tuned to the amplitude-modulation intermediate frequency.The series impedance of each of the component tuned circuits isnegligible at the intermediate carrier frequency to which it is notintended to respond so that the tuned circuits eiectively comprise anautomatic switching arrangement for selectivelyreceiving either theamplitude-modulated or the frequency-modulated intermediate-frequencysignals.

The receiver also includes -a' dual-purpose signal-translating stagecomprising a vacuum tube- I having a plurality of electrodes, includinga signal input electrode or grid |5a andy `a signal output electrode oranode |5b. Specifically, the tube is illustrated as of the pentode typeincluding also conventional screen and suppressor electrodes. Thesecondary circuit of the selector I 4 Vcomprises an input circuit forapplying received intermediate-frequency signals to the input electrodeI5a of tube l5 and includes a timeconstant circuit such as a seriescondenser I6 shunted by a leak. or load resistor I1 therefor. The anode|51) of tube I 5 and its screen are energized from a suitable sourceindicated as +B through a dropping resistor I8 by-passed' to ground bymeans of a condenser I9 shunted by a resistor 20.

The receiver also includes a frequency discriminator 2| coupled to theoutput electrode I 5b of tube l5. The discriminator may be of anyconventional form but, as illustrated, comprises a primary tunedVcircuit 2|a coupled to a secondary tuned circuit comprising an inductor2lb shunted by two series-connected condensers 2|c and 2Id. The inductor2lb is inductively coupledtothe primary circuit 2|a and, as indicatedschematically, is adjustably tuned by means of a powdered. iron or-likeferro-magnetic core 2|Ic. The high. signal-frequency-potential terminalof the circuit 2|a is connected to thejunction between condensersZIc and2|d. Terminals ofthe secondary tunable. circuit 2lb, 2|c, 2|d areconnected` todiode rectiiiers 2|e and. 2| f shunted by load. resistors.2Ig and 2I`h, respectively, the cathode Yof the diode V2 If beinggrounded. and the cathodev of the diode 2|e being by-passed to groundfor intermediate-frequency signals through a condenser 2li. Thesignal-.frequency output is derived from the terminal-2li of thediscriminator 2|. With the circuit connections forthe tube I5 asdescribed, it operates as a conventional frequency-modulation amplifierand limiter.

The stage also includes circuit connections for the' electrodes of tubel5 such. that it operates with the time-constant circuit IB, l1 as anamplitude-modulation detector. To this end there is provided switchmeans operable in its first position to modify the normal tube electrodeconnections andto establish the amplitude-modulation detector circuitconnections. Specifically, this means comprises a two-position switch 22effective-in its first position, as illustrated, to connect the screenand output electrode or anode of tube I5 to ground, so that the inputelectrode |5 a and cathode ci the tube I5 `function as a diode.amplitude-modulationdetector with the timeconstan-t circuit I6, II asits load circuit. There is also vprovided a two-position switch 23operable time-constant circuit I6, to a signal output circuit 24 of thereceiver. The switches 22 and 23 are ganged for unicontrol with eachother and with the switches Illa, I2a of the units I0 and I2 by means ofa mechanism indicated schematically at U. The switch 22 is operable inits second or FM position to remove the ground connection from thescreen and output electrodes of tube l5V and to restore the tubeelectrode connections to normal; that is, to establish the FM limitercircuit connections. Simultaneously, the switch 23 inA its secondrposition is effective to connect the output circuitv 24 to the terminal2li of the discriminator 2|y through a filter comprising a seriesresistor 25v and shunt condensers 2|z and 26. The values of the'resistorand condenser I6 of the time-constant circuit are chosen to impart ashort time-constant of the order of 2 microsecondsy to this circuit,which is necessary both for good limiter action and for satisfactoryamplitude-modulation detection. The negative bias developed across thecircuit I6, is applied as an automatic ampliiication control bias to thecontrol electrodes of lone or more of the tubes included within theunits I0, I2 and I3 by way of the AVC circuit, as indicated.

The receiver also includes an audio-frequency amplifier 2l coupled tothe signal-output circuit 24 and havingan output circuit connected to asignal reproducing means, such as a loud speaker 28.

The. operation of the complete receiver illustrated in the drawing willbe readilyv understood from the foregoing description. In brief,however, the units I0 and I2 are effective to derive from a signalpicked up by the antenna circuit. I I an intermediate-frequency signalhaving one value of intermediate carrier frequency forfrequency-modulated signals and a substantially different, usually lowervalue, of intermediate carrier frequency for amplitude-modulatedsignals, as. determined. by the constants Aof the oscillator circuitwhich are adjusted b-y operation of the switch I2a.

When the switches |0a, |2a, 22 and 23 are operated to their right-handpositions for the reception of frequency-modulated signals, theintermediate-frequency signals derived therefrom. are selected bytheportions Illa, Nb ofv selector I4 and applied to the signal-inputelectrode of tube l5.V Due to thev self-biasing, action of the gridcircuit of this tube, including the time-constant circuit I6, I'I, andto the selection of its screen and anode potentials, the tube I5operates in a conventional manner as a limiter to develop in its outputcircuit intermediate-frequency signals of substantially constantamplitude but frequency-modulated in accordancewith the transmittedsignal. This intermediate-frequency signal 'is impressed upon thediscriminator 2 which operates in a conventional manner to develop atits terminal 2 la' a signal-frequency output. While the operation of thediscriminator 2| is conventional, its. circuit is not entirelyconventional in that the primary voltage is fed into the. secondarycircuit 2| b, 2Ic and 2|d at the junction between the condensers 2|c and2id-...which effectively comprise a voltage divider, rather than at amidtap of the inductor 2lb as is conventional. This arrangementpermitsthe use of aninductor 2lb without an. electrical midpoint and thereforeadapted for tuning by the. use of an. adjustable powderediron or otherferromagnetic core. Such an arrangement has well known. improvedfrequency stability characteristics.

The signal-frequency output fromy the terminal 2 Iy is-passed throughthe'carrier frequency 23 upon the audio-frequency ampliiier 21 fromwhich it is delivered to the loud speaker 28 for reproduction. i .y v fVOn the other hand, ifL the switchesy Ia, I2a, 22 and 23 are operated totheir-left-hand positions; as illustrated in the drawing, anintermediate-frequency `signalof lower carrier frequency is developedbyunit I3 and `selectedin portions I4c, I4dbf selector 'I4 and impressedupon the signal-input electrode'v I 5a of Y-tube I5.

With this connection, the screen and anode electrodes of tube I5 areeffectively grounded so that the vgrid and cathode of'.this'tubeioperate With the time-constant circuit I6; I1 asa'conventional diode peak detector, developing a signal-frequency outputacross the time-constant circuit This output is impressed through theswitch 23 upon the audio-frequency ampliiier 21 from which it isdelivered to the loud speaker 28 for reproduction. l

As is well understood in the art, the AVC connection from thetime-constant circuit I6, I1 is effective to impress upon the controlelectrodes -of the tubes of the units I Il, I2 'and I3 a negative biaspotential proportional tothe average intensity of the received carriersignal and thusV to control the amplification in these units in Suche.Way as to maintain the amplitude of the signal input to detector I5within a narrow range for a wide range of received signal intensities.V

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be apparent to thoseskilled in the art that various changes and modifications may be madetherein without departing from the spirit or scope of the invention.

I claim:

l. In a wave-signal receiver adapted selectively to receiveamplitude-modulated signals or frequency-modulated signals, adual-purpose signal-translating stage comprising, a vacuum tube having aplurality of electrodes includin-g a signal input electrode and a signaloutput electrode, an input circuit for applying received signals to saidinput electrode, a frequency discriminator coupled to said o-utputelectrode, a signa1-frequency output circuit, circuit connections forsaid tube electrodes to cause said tube to operate as a diodeamplitude-modulation detector, circuit connections for said tubeelectrodes to cause said tube to operate a's a frequency-modulationlimiter, and switch means operable in a iirst position to establish saidrst-named circuit con-V nections and to connect said output circuit tosaid amplitude-modulation detector circuit and operable in a secondposition toV establish said second-named circuit connections and toconnect said output circuit to said discriminator.

2. In a wave-signal receiver adapted selectively to receiveamplitude-modulated signals or frequency-modulated signals, adual-purpose signal-translating stage comprising, a vacuum tube having aplurality of electrodes including a signal input electrode and a signaloutput electrode, an input circuit for applying received signals to saidinput electrode including a timeconstant circuit, a frequencydiscriminator coupled -to said output electrode, a signal-frequencyoutput circuit, circuit connections for said tube electrodes to causesaid tube to operate with said time-constant circuit as a diodeamplitude-modulation detector, circuit connections for said tubeelectrodes to cause said tube to operate-asa Ifrequency-modulationlimiter, andv switch means operable in a iirst position to establishsaid iirstnamed circuit connections and to connectsaid output circuit tosaid time-constant circuit and operable in a second position toestablishfsaid V second-'named circuit connections and to connect saidoutput circuit to said discriminator.

3. In a wave-signa1 receiver adapted selectively to receiveamplitude-modulated signals or irequency-modulated signals, adual-purpose signaltranslating stage comprising, a vacuum tube having aplurality of electrodes including a signal input electrode and a signa1output electrode,` an input circuit for applying received signals tosaid input electrode including a series condenser and a leak resistortherefor, a frequency. discriminator coupled to said output electrode, asignal-frequency output circuit, circuit connections for said tubeelectrodes to cause "said tube to operate with said time-constantcircuit as a diode amplitudemodulation detector,l circuit connectionsfor said tube electrodes to cause said tube to operate as afrequency-modulation limiter, andA switch means'operable ina iirstposition to establish said iirst-named circuit connections and toconnect said output circuit to said condenser and operable in a' secondposition to establish said secondnamed circuit connections and toconnect said output circuit to said discriminator.

4. In a wave-signa1 receiver adapted selectively to receiveamplitude-modulated signals or frequency-modulated signals, adual-purpose signaltranslating stage comprising, a vacuum tube having aplurality of velectrodes including a signal input electrode and a signaloutput electrode, an input circuit for applying received signals to saidinput electrode including a time-constant circuit, a frequencydiscriminator coupled to said output 40 electrode, a signal-frequencyoutput circuit, said to receive amplitude-modulated signals or fre-Vquency-modulated signals, a dual-purpose signaltranslating stagecomprising, a vacuum tube having a plurality of electrodes including asignal input electrode and a signa1 output electrode, an input circuitfor applying received signals to said input electrode including atime-constant circuit, a frequency discriminator coupled to said outputelectrode, a signal-frequency output circuit, said tube electrodesnormally being connected to operate as a frequency-modulation limiter,and switch means operable in a rst position to connect said signaloutput electrode to ground to cause said tube to operate with saidtime-constant circuit as a diode amplitude-modulation detector and forconnecting said output circuit to said time-constant circuit andoperable in a second position to restore said tube electrode connectionsto normal and to connect said output circuit to said discriminator.

6. A wave-signal receiver adapted selectively to receiveamplitude-modulated signals or frequency-modulated signals comprising, aradiofrequencyselecting Vvandxconverting means for de.. riving' from aselected receiveclradio-irequency signal an intermediate-frequencysignal, a dualpurpose` signal-translating stage including a Vacuum tubehaving'a plurality of electrodes includingza signal input electrode anda signaloutput electrode, an input circuit'for applying received signalsto said input electrodev including a timeconstant circuit, a frequencydiscriminator'coupled to. said output electrode;` a. signal-frequencyoutput circuit, circuit connections for saidtube electrodes to cause`said tube -tooperate with said time-constant circuit as a diodeamplitude-modulation detector, circuit connectionsI for said tubeelectrodes to cause ysaid tube-to operate as a frefluency-modulationlimiter, and switch means-operable in-a rst position toestablish saidiirstnamed circuit connections and to connect said output circuit tosaid time-constant-circuit and operable in a secondposition to establishsaid second-named circuit connections and to connect said output circuitto said discriminator, anda signal-reproducing means coupled 4tosaidoutput circuit.

7. A Wave-signal receiver adapted selectively toreceiveamplitude-modulated signals or frequency-modulated signalscomprising, a radiofrequency selecting and converting meansfordetime-constant circuit as a diode amplitude-modulationdetector, circuitconnections for said tube electrodes toA cause said tube to operate as afrequency-modulation limiter, and switch means operable in a rstposition to establish said firstnamed circuit connections and to connectsaid output circuit to said time-constant circuit and operable inasecond position to establish said second-named circuit connections andto connect said output circuit to said discriminator, switch meansincluded in said selecting and converting means for selecting signalsfrom the amplitudemodulation range or the frequency-modulation range,unicontrol operating means for both of said switch means, and asignal-reproducing means coupled to said output circuit.

JOHN Wa HUFF.

